John 1:1 - "In the beginning was the Word, and the Word was with God, and the 
Word was God." John 1:2-3 goes on to read "He (Jesus) was in the beginning with 
God. All things were made through him, and without him, nothing was made."

I find this fascinating and very relevant to everything we, as Christians, 
believe. Genesis 1:1 begins by "In the beginning, God".  John 1:1 confirms this 
verse in its beginning statement "In the beginning was the Word".  John 1:1 
doesn't say "In the beginning was God, God had the Word."  Instead, John 1:1 
connects Jesus to God.  Jesus was there in the beginning and is still there now.  
John 1:1 confirms the second person the Trinity - God the Son. 

The preexistence of Jesus means he is fully God. He is eternal. 
John 8:58 - "Jesus said to them, 'Truly, truly, I say to you, before Abraham 
was, I am."

John 13:19 - "I am telling you this now, before it takes place, that when it 
does take place you may believe that I am He."  In this scripture, Jesus washed 
his Disciple's feet and spoke to them about the fulfillment of the prophesies 
and the Scriptures.  The disciples were able to see first hand of Jesus' 
lashings, his beatings, his crucifixion and finally his resurrection.  However, 
I believe Jesus is alluding to something more.  I believe He is talking about 
his return. 

John 18:6 - "When Jesus said to them, 'I am he', they drew back and fell to the 
ground."  Falling to the ground is a common reaction to divine revelation.  This 
is confirmed in Revelation 1:17, Ezekiel 1:28, Daniel 2:46, 8:18, 10:9, Acts 9:4 
just to name a few. 

Phillipians 2:5-11  

Colossians 1:15-18

However, there are some arguments.  When turning to Matthew 1:1, it begins with 
the genealogy of Jesus Christ and states..."the Son of David, the Son of 
Abraham."  By this scripture, some would argue that Jesus did not preexist.  
However, God had to fulfill the Scriptures of Isaiah 9:7, 11:1 and 11:10.  God 
did this also to show the genealogy from Abraham to David and finally to Jesus. 
Another argument is that Jesus was born to Mary (Luke 1:26-31).  Once again, the 
prophesy in Isaiah 7:14 had to be fulfilled... born to a virgin.  

The preexistence of Jesus confirms the Trinity.  

As far back as one wishes to push the "beginning", the Word was already created. 
It already existed. 

Blessings in Jesus Mighty Name,